U.S. Pat. No. 5,242,402 to Long-Hsiung Chen disclosed a safety syringe for intravenous injection including: an annular groove annularly recessed in a front surface of a plunger slidably held in the syringe to be engageable with a needle head portion formed on a rear portion of a hollow needle eccentrically mounted in a front portion of the syringe, with the annular groove having a longitudinal section formed as a biasing socket engageable with the needle head portion for biasing the needle obliquely within the syringe when retracting the plunger and the needle into the syringe, thereby preventing an outward protruding of the retracted needle for safety and hygienic purpose.
However, such a conventional safety syringe may have the following drawbacks:
1. When engaging the needle head portion 23 of the needle device 2 with the biasing socket of annular groove 32 of the plunger 31 for retracting a used needle into the syringe, the needle head portion 23 may still be released from the annular groove 32 to be accidentally protruded outwardly through the tunnel 130 since the groove 32 is concentrically recessed in the plunger 31, not a single round socket for tightly engaging the needle head portion 23.
2. After retracting the needle in the syringe cylinder, the plunger may still be rotated or twisted to re-protrude the retracted needle outwardly to cause dangerous pricking since the plunger 31 can be free rotational about the syringe axis 100 within the syringe means 1.
3. In order to repel the liquid medicine filled in the annular groove 32, the extension 15 must be provided on a rear surface of the front plug 13 for engaging the groove 32. If the extension 15 is engaged with the groove 32 too tightly, the plug 13 as coupled with the plunger 31 straightly holding the needle 2 will be together retracted inwardly into the cylinder 11, without biasing the needle obliquely.
4. The needle 2 should be inserted through the plug in the syringe cylinder to be mounted on the syringe to cause inconvenience. Also, it can not be replaced with needles of different sizes from outside the syringe.
The present inventor has found the drawbacks of the conventional safety syringe and invented the present safety syringe with externally connectable needle.